1. Field of the Invention
This invention relates to vertical shaft solids continuous melting furnaces wherein solid feeds are charged to the top forming a bed of feed materials supported by a cooled grid. The charged solids move downwardly in the bed and are contacted by countercurrent hot combustion product gases causing intensive preheating and substantial melting of the solids above the cooled grid. The melt flows downwardly through the lower portion of the bed and through the grid, with some small solids, into a submerged fired or oxidizing gas blown melt pool at the bottom of the vertical shaft melting furnace. The vertical shaft solids melting furnace may be combined with a number of melt treating configurations. Conventional fossil derived fuels or organics containing charge may be used to result in higher combustion, melting and melt treating intensities with higher production rates, higher thermal efficiencies, and lower capital and operating costs. The product provided by the apparatus and process of this invention is of high quality and without contaminants as resulted from many prior melting and melt treating processes.
2. Description of the Prior Art
Conventional, commercially used melting and melt treating furnaces include principally reverberatory furnaces and cupolas. Reverberatory furnaces have been very expensive in capital cost and operation, providing low specific production rates and low thermal efficiencies. While cupolas have been less expensive and have higher specific production rates, they have used expensive coke and frequently the quality of the melt is relatively low due to poor homogeneity and temperature fluctuations while emissions of carbon monoxide, unburned hydrocarbons, and hydrogen sulfide have been higher than desired. There has been some testing of new approaches using rotary, cyclonic and other flash type melters as well as electric melters, but these have not proved entirely satisfactory for many industrial applications. A major drawback of these new approaches using fossil fuel is the absence of satisfactory reliable heat recovery systems necessary to achieve high thermal efficiency of the furnace. All of the heat recovery systems known to the applicant to have been tested with such furnaces failed to operate reliably in flue gas streams highly loaded with particulates, molten droplets and vapors.
A process and apparatus for melting of pig iron in a rotary furnace is illustrated by U.S. Pat. No. 4,101,313. U.S. Pat. No. 4,140,480 teaches a cupola having alternate layers of charge and coke with a flue gas bypass from the lower portion of the bed to the headspace providing self-ignition of carbon monoxide. Preheating of charged solids in a cupola by passing combustion gases or flue gases in contact with the charged solids is exemplified by U.S. Pat. Nos. 4,605,437; 4,203,761; 3,788,832; 3,424,573; 3,169,015; 3,157,492; and 1,713,543. It is also known to charge metallic solids into a vertical chamber and conduct the melting by flames at the bottom of the solids column, the solids being supported by an arch of the solid material welded together above the flame at the bottom as taught by U.S. Pat. Nos. 4,110,108; 4,097,028; and 3,948,642. It is also known to support the solid particles to be melted in a cupola on top of a refractory bed which is supported by a water cooled grid as taught by British Pat. Nos. 1,326,884 and 975,569. Submerged combustion melting of glass batch and cullet is known wherein the raw material is supplied directly to the liquid glass in the melting chamber and submerged combustion is effected by burners completely submerged under the molten glass to effect the melting as taught by U.S. Pat. No. 3,260,587. One disadvantage of prior submerged combustion processes has been lower furnace thermal efficiency than desired.
Problems encountered with the prior art fossil fuel fired cupola melting using coke admixed with the charged solids have been that melt quality has not been as high as desired due to coke contamination; the melt has not been as homogeneous as desired; nor has the melt temperature been controllable within ranges desired. In addition, prior art cupola melting has produced undesired amounts of carbon monoxide, unburned hydrocarbon, and hydrogen sulfide emissions. There have been attempts to develop cokeless cupolas which have been natural gas fired and have water cooled grids with a coke bed on top of the grids to support the solids and to allow melt to flow through to a melt pool. These have not proved entirely satisfactory since there is still undesired contamination to the melt by coke particles falling through the support grid into the melt and there are still undesired carbon monoxide, unburned hydrocarbon, and hydrogen sulfide emission problems. Attempts to substitute a refractory bed for the coke bed as a solids support on top of the water cooled grid has not been entirely satisfactory since refractory particles drop into the melt contaminating it and, further, it has been necessary to replace such refractory beds more frequently than desired.